gallium/xlib: Partial fix for glXCopySubBufferMESA

xmesa_copy_st_framebuffer would attempt to flush from the front buffer
to the display, but we don't actually have an attachment for the front
buffer (just the back) so nothing would happen. Fix this by flushing
fron the back to the display, threading the dirty box through so we
don't update more than we were told to.

This has the virtue of displaying correctly, but glx-copy-sub-buffer
still fails since there is no real front buffer, reads from GL_FRONT
actually read from the back buffer. The test does: clear to red, swap,
clear to green, copy sub-buffer, expect a green square inside of a red
one from the front buffer. Since you're really reading from the back you
instead get solid green.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9140>

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